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| Titel |
Physico-chemical transition from peridotite assemblage to the eclogite one (experimental data at 7.0 GPa). |
| VerfasserIn |
Valentina Butvina, Yurii Litvin |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250034742
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| Zusammenfassung |
| Peridotites and eclogites, including diamond-bearing ones, are the basic ultra-basic and basic
rocks of the upper mantle (Ringwood, 1969, 1975; Sobolev, 1974; Marakushev, 1985; Taylor
& Anand, 2004). These rocks are presented in the assemblage of mantle xenolyths in
kimberlites, but the basic minerals of peridotite paragenesis, olivine, orthopyroxene, garnet
and clinopyroxene as well as of an eclogite paragenesis, garnet and omphacite are
wide-spread synthetic inclusions in diamonds. The cases of finding minerals and peridotite
and eclogite parageneses in diamond are described. It implies that these parageneses can have
a single mantle source. However, the formation of peridotite and eclogite mineral parageneses
at differentiation of the primary ultrabasite melt during physico-chemical single process is
possible only at overcoming the “eclogite” thermal barrier (O’Hara, 1968; Litvin,
1991).
Eclogite genesis is one of the most difficult and discussional problems of modern
petrology. Among investigators there is an opinion about eclogite heterogeneity
not only on conditions of formation (crust, mantle), but also by composition of
the initial rocks (para-, orthoeclogites) as well as by the way of their formation
(magmatic, metamorphic, metasomatic). In literature diamond-bearing eclogite nodules of
kimberlite pipes are often considered as metamorphic, which are formed at subduction
of the Archean or of the Proterozoic oceanic crust (MacGregor & Manton, 1986;
McCandless & Gurney, 1986, 1997 et al.). Only the presence of Na2O in garnet
and K2O in clinopyroxene is a criterion of their participation in mantle magmatic
processes.
Together with the hypotheses considered on eclogite origin there exists a version
suggested in papers (Kushiro, 1972; Kushiro & Yoder, 1974), according to which mantle
eclogites could be formed due to peridotite substance in the processes of fractional
crystallization of ultrabasite magmas. The present paper is devoted to the experimental study
of this problem.
Physico-chemical transition from peridotite assemblage to the eclogite one can be only
ensured by the processes of fractional crystallization of mantle magmatic melts. The primary
melting and magmatic evolution of mantle garnet lerzolite (or the Ringwood pyrolite) is
controlled by a five-phase peritectics “p” Ol+Opx+Cpx+Grt+L and four cotectic curves
conjugated to it (Litvin, 1991). In melting and evolution of melts of both olivine eclogites and
coesite and corundum eclogites the corresponding five-phase eutectics are of a dominant
importance. A general ridge for all elementary tetrahedrons (simplexes) is a line of
compositions diopside-pyrope (clinopyroxene-garnet) which bimineral eclogite
assemblages belong to. The internal section En-Di-Cor of the general tetrahendric diagram
(symplex complex) separates olivine-saturated and silica-saturated compositions.
“Eclogite” thermal barrier is “thermal barrier” on (O’Hara, 1968), on the cotectic
line Opx+Cpx+Grt+L, connecting “peridotite” peritectic and “eclogite” eutectic
points.
Meanwhile, at equilibrium (and fractional) crystallization of peridotite system in the
peritectic point “p” orthopyroxene vanishes as a result of the peritectic reaction “orthopyroxene +
melt - clinopyroxene” (Davis, 1963; Litvin, 1991). With further temperature decrease the
composition of the remnant melt is controlled by the nonorthopyroxene cotectics
Ol+Cpx+Grt+L first, in the limits of the peridotite “simplex”, but then mechanism of
fractional crystallization is also realized in the limits of the olivine-eclogite “simplex” up to
the corresponding nonvariant eutectics.
The considered cotectics Ol+Cpx+Grt+L is of the greatest interest from the viewpoint of
a possible change of compositions of remnant melts from olivine-normative to
silica-normative ones. One can assume that under the conditions of fractional melt
crystallization along the cotectic curve Ol+Cpx+Grt+L together with olivine jigging
accumulation of incorehent elements, including Na, Fe etc. takes place. It leads to a gradual
increase of jadeite component content in remnant melts what creates grounds for reactional
interaction of jadeite and olivine components with olivine vanishing and garnet formation in
accordance with the reaction found in (Litvin et al., 2004). A gradual decrease of olivine
component content in remnant melts caused by that fact realizes a “turn” to the cotectic curve
Ol+Cpx+Grt+L in the direction of the boundary section En – Di – Cor and, probably
its exit to the line Di–Prp (clinopyroxene-garnet). Further under the conditions of
fractional crystallization melt composition point can penetrate into the volumes of
coesite-eclogite, kyanite-eclogite and corundum-eclogite “symplexes”. Thus, an
overcoming of “eclogite” thermal barrier between olivine-normative peridotite-pyroxene
and SiO2 – normative eclogite compositions occurs. So, one can speak about the
“destruction” of liquidus peridotite-eclogite thermal barrier in the limits of the peridotite
“simplex” as a result of realization of two reaction mechanisms: (1) vanishing of
orthopyroxene as a result of its peritectic reaction with the melt with clinopyroxene
formation and (2) olivine vanishing as a result of its reactional interaction of jadeite
with garnet formation. If with respect to the first mechanism definite experimental
evidence exists (Litvin, 1991; Davis, 1963) then for the second mechanism it is
absent.
Due to this fact the main purpose of this paper is an experimental study of phase
relationships in the model system forsterite-dioside-jadeite at pressure of 7 GPa and
foundation of possible physico-chemical correct transitions between peridotite and eclogite
parageneses with overcoming liquidus “eclogite” thermal barrier. To construct a
diagram of a ternary system forsterite-diopside-jadeite it is necessary to study its
boundary binary sections forsterite-jadeite and fosterite-diopside as well as a number of
internal polythermic sections. The section jadeite-diopside at 7 GPa has been studied
earlier (Bobrov, Litvin, Kojitani, Akaogi, 2006; 2008) and it is characterized by
the unlimited miscibility of jadeite and diopside components in solid and liquid
states.
The first experimental results obtained at the initial stage of the investigation of this
problem can be characterized as follows.
For the experimental study polythermic sections of forsterite-(jadeite50diopside50) and
forsterite-(jadeite25diopside75) have been chosen. The obtained data testify to the fact that
olivine vanishing and garnet formation are realized in both sections. The problem of further
investigations is to search minimum concentrations of jadeite in the composition of this
system where a total olivine vanishing takes place.
Thus, the performed experimental investigations of the model system
forsterite-diospside-jadeite at pressure 7 GPa testify to the fact that forsterite (olivine) is a
stable phase in the boundary system forsterite-diopside (olivine-clinopyroxene). While
introducing rather low contents of jadeite component into the composition of this system the
reaction of jadeite component with forsterite takes place in the melt. As a result, garnet
appears as liquidus phase. |
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